1. Field of the Invention
This invention relates to high power, infra-red, optically pumped CO2 lasers.
2. Description of the Prior Art
U.S. Pat. No. 3,810,042 (Tunable Infrared Molecular Laser Optically Pumped by a Hydrogen Bromide Laser) describes lasers that are disclosed, wherein there are laser configurations in which a high pressure molecular medium such as compressed CO2, liquid CO2 or solid CO2 is pumped by a hydrogen-bromide molecular laser. The hydrogen-bromide laser, when transversely excited, can provide high pump powers between 4.0 and 4.6 micrometers wavelength. This wavelength range is desirable for pumping molecules having a linear three-atomic skeletal, including those molecules which are simple linear three-atomic molecules. An example of a more complicated molecule having a linear three-atomic skeletal is CH2CO. The active medium has a spectrum of vibrational-rotational emission lines that merge into a continuum that is suitable for mode-locked pulsing or tunable operation.
The primary deficiency is the use of a Hydrogen Bromide (HBr) laser molecular pump which, in practice, is not suitable. Chemical lasers, though in general efficient, require the handling of exhaust product and precursor fuel and oxidizer. Even if purely cold-reaction discharge initiated by dissociation of a halogen donor, these are then typically inefficient and still require product gas handling. In addition, HBr is a multiline molecular laser with a defined emission line structure, thus at approximately atmospheric pressure or less, the Carbon Dioxide (CO2) attributable absorption line structure is only in sporadic proximity to a limited number of HBr lines, both CO2 and HBr of limited bandwidth, and thus efficient pump utilization is not easily achieved under such conditions. That is to say, HBr is not continuously tunable within a broad band, thus as a pump for CO2 it offers, at low to moderate pressure, no more than limited line matching. In addition, the 1→0 vibrational transition of HBr occupies the spectral range 2400 cm−1 to 2700 cm−1, which presents essentially no spectral overlap with the 00°0→00°1CO2 transition being pumped. The forgoing has direct implications in regards baseline useful emission, for any isotopologue of CO2, from HBr, and thus global system efficiency whether or not CO2 is at pressure. Further deficiencies, common to this cited prior art and those cited below are presented in the final paragraphs of this section. By convention the vibrational band of CO2 is designated by a numerical term of the form abcd, where ‘a’, ‘b’ and ‘d’ are the v1, v2 and v3 vibrational quantum numbers respectively. ‘c’ is the angular momentum quantum number. The symbol → denotes a transition, and can be read as ‘to’. Isotopologues are molecules that differ only in their isotopic composition.
U.S. Pat. No. 3,860,884 (Optically Pumped N2O or similar gas mixed with Energy Transferring CO2) describes a powerful excitation technique for a mixture of molecular gases in which a combination of optical pumping and resonant energy transfer is used. An optically-pumped N2O laser pumped at 4.3 μm by HBr laser is “seeded” with a minor portion of CO2, which absorbs the pumping radiation and transfers it by vibration-vibration energy transfer to invert the populations of the 00°1 and 10° levels of the N2O. In this laser oscillations have been achieved at 10.5 μm at total pressure up to 42 atmospheres, which is more than an order of magnitude greater than feasible in an optically-pumped N2O laser without CO2. The advantages of broad tunability and short pulse width are obtainable. In addition, a rare isotope CO2 laser employs 13C16O2 to comprise at least 90 percent and possibly as much as 97 percent of the gas mixture, together with as little as 3 percent of ordinary CO2 (12C16O2). Only the ordinary CO2 absorbs a significant portion of the pumping radiation directly; but significant energy transfer occurs by collision from the ordinary CO2 to the 13C16O2. Laser oscillation is thereby obtainable between about 9 μm and 11 μm for a total pressure exceeding about 40 atmospheres.
The primary deficiency is, as per the prior cited patent, the use of a Hydrogen Bromide (HBr) laser pump which, in practice, is not suitable. Chemical lasers, though in general efficient, require the handling of exhaust product and precursor fuel and oxidizer. Even if purely cold-reaction discharge initiated by dissociation of a halogen donor, these are then typically inefficient and still require product gas handling. In addition, the 1→0 transition of HBr occupies the spectral range 2400 cm−1 to 2700 cm−1, which presents essentially no spectral overlap with the 00°0→00°1 CO2 transition being pumped, for any isotopologue of CO2. The forgoing has direct implications in regards baseline useful emission from HBr, and thus global system efficiency whether or not CO2 is at pressure. Further deficiencies, common to this cited prior art and those cited below are presented in the final paragraphs of this section.
U.S. Pat. No. 4,145,668 (Optically Resonance Pumped Transfer Laser with High Multiline Photon to Single Line Photon conversion efficiency) describes lasers that are disclosed, wherein trapped multiline laser radiation from a DF laser is employed to pump a DF+CO2 working gas mixture within the optical resonator for the DF laser. The multiline pumping energy is resonantly absorbed by the DF component of the working gas mixture and collisionally transferred to upper energy levels of lasing transitions in CO2. A narrow-band optical resonator disposed about the working gas interaction region with the pumping radiation and tuned to a desired CO2 transition enables a single line laser output to be obtained on the desired transition.
The primary deficiency is, as per the prior cited patents, the use of a chemical laser pump which, in practice, is not suitable. In this case, the specified Deuterium Fluoride (DF) laser pump is not practical. Chemical lasers, though in general efficient, require the handling of exhaust product and precursor fuel and oxidizer. Even if purely cold-reaction discharge initiated by dissociation of a halogen donor, these are then typically inefficient and still require product gas handling. Further deficiencies, common to this cited prior art and those cited below are presented in the final paragraphs of this section.
The article, New Direct Optical Pump Schemes for Multi-atmosphere CO2 and N2O Lasers [K. Stenersen et al, IEEE Journal of Quantum Electronics, 25(2), 1989, 147-153] describes nine schemes for direct optical pumping of multi-atmosphere CO2 and N2O lasers at pump wavelengths in the 1.4-3.6-μm region. Most of these wavelengths can be generated by solid-state lasers, which are more attractive pump sources than the chemical lasers (HBr, HF) used previously to pump high-pressure CO2 and N2O lasers. Including previously studied pump schemes, there are altogether 14 possible pump transitions in CO2 and N2O in the 1.4-4.5-μm region. Numerical laser simulations are carried out to compare all of these pump schemes. Assuming 10 J/cm2 pump energy in a pulse of 100 ns FWHM, and 5% output coupling as the only resonator loss, the calculated energy conversion efficiencies are in the range of 6-40%. The pump thresholds are in the range of 0.1-3.1 J/cm2.
This article correctly identifies the 00°0→20°1 transition as a possible CO2 optical pump path but it is deficient in that related identified solid state, for example, Cobalt doped Magnesium Fluoride (Co:MgF2) and Erbium doped Yttrium Lithium Fluoride (Er:YLF), optical pump systems are simply not sensible. At room temperature the Co:MgF2 upper state lifetime is ˜35 μs. The symbol ˜ denotes approximately throughout this text. Thus Co:MgF2 can only be pumped, at room temperature, by another pulsed laser, perhaps laser diode excited. Global efficiency will diminish for each new laser inserted in the system chain, as will complexity, thus an undesirable concept. To date, Neodymium doped, Yttrium Aluminum Garnet (Nd:YAG) at 1338 nm has been used to pump Co:MgF2, and Nd:YAG on this line is no more than ˜24% efficient, and pumped Co:MgF2 was no more than ˜35% efficient to yield a net best pump source efficiency of ˜8%. Since the primary laser diodes pumping Nd:YAG would be ˜55% efficient the optical to electrical efficiency of this pump system would be ˜5%. This as opposed to the ˜25% or better feasible for the laser diode pumped Thulium doped, Yttrium Aluminum Garnet (Tm:YAG) for example. In the case of Erbium doped (say Er:YLF) lasers, they simply cannot efficiently get into the correct pump band around ˜2 μm for CO2. The article is deficient in that it does not identify the 0111 level as a possible, beneficial, metastable level deriving from the identified pump transition. Other deficiencies are presented in the final paragraphs of this section.
The article, Direct Optically Pumped Multiwavelength CO2 Laser [M. I. Buchwald et al, Applied Physics Letters, 29(5), 1976, 300-302] describes an HF laser that was used to directly pump various isotopic forms of CO2. Intense laser emission was observed on numerous lines in the 4.3, 10.6, and 17 μm regions. All observed 4.3 and 17 μm CO2 laser emission lines were assigned. The pressure dependence of lasing spectra and laser pulse temporal features were examined.
This approach is deficient in that optical pumping of CO2 on the 00°0→10°1 transition was employed. This required a Hydrogen Fluoride (HF) laser which is not sensible from a practical standpoint. As per the prior cited patents, the use of a chemical laser pump, which in practice, is not suitable. In this case, the specified Hydrogen Fluoride laser pump is not practical. Chemical lasers, though in general efficient, require the handling of exhaust product and precursor fuel and oxidizer. Even if purely cold-reaction discharge initiated by dissociation of a halogen donor, these are then typically inefficient and still require product gas handling. In addition, it is of course a multiline laser and there is only coincidental approximate line matching of some form and the 1→0 transition offers no meaningful spectral overlap with the 00°0→10 °1 pump transition. All of which has direct implications in terms of net efficiency and suitability as a pump for molecular CO2. Further deficiencies, common to this cited prior art and those cited below are presented in the final paragraphs of this section.
The article, Optically Pumped Atmospheric Pressure CO2 Laser [T. Y. Chang et al, Applied Physics Letters, 21, 1972, 19] describes laser action at 10.6 μm that has been obtained in pure CO2 gas at pressures up to 1 atm by optically pumping with the 4.23 μm line of a TEA HBr laser. The potential usefulness of this method for pumping very high density CO2 is discussed.
The primary deficiency is, as per the prior cited patents and article, the use of a Hydrogen Bromide laser pump which, in practice, is not suitable. HBr is a multiline molecular laser with a defined emission line structure, thus at approximately atmospheric pressure or less the CO2 attributable absorption line structure is only in sporadic proximity to a limited number of HBr lines, both CO2 and HBr of limited bandwidth and thus efficient pump utilization is not easily achieved under such conditions. That is to say, HBr is not continuously tunable within a broad band, thus as a pump for CO2 it offers, at low to moderate pressure, no more than limited line matching. In addition, the 1→0 transition of HBr occupies the spectral range 2400 cm−1 to 2700 cm−1, which presents essentially no spectral overlap with the 00°0→00°1 CO2 transition being pumped. The forgoing has direct implications in regards baseline useful (for any isotopologue of CO2) emission from HBr, and thus global system efficiency whether or not CO2 is at pressure. Furthermore chemical lasers, though in general efficient, require the handling of exhaust product and precursor fuel and oxidizer. Even if purely cold-reaction discharge initiated by dissociation of a halogen donor, these are then typically inefficient and still require product gas handling. Further deficiencies, common to this cited prior art and those cited below are presented in the final paragraphs of this section.
The article, Optically Pumped N2O Laser [T. Y. Chang et al, Applied Physics Letters, 22, 1973, 93], describes laser action at 10.8 μm that has been obtained in pure N2O gas at pressures up to 270 Torr by optically pumping with the 4.465 μm line of a TEA HBr laser. Possible extension of this pumping technique to other linear three-atomic molecules and molecules with a linear three-atomic skeletal is discussed.
The primary deficiency is, as per the prior cited patents and articles, the use of a Hydrogen Bromide laser pump which, in practice, is not suitable. Chemical lasers, though in general efficient, require the handling of exhaust product and precursor fuel and oxidizer. Even if purely cold-reaction discharge initiated by dissociation of a halogen donor, these are then typically inefficient and still require product gas handling. In addition, HBr is a multiline molecular laser with a defined emission line structure, thus at approximately atmospheric pressure or less the N2O attributable absorption line structure is only in sporadic proximity to a limited number of HBr lines, both N2O and HBr of limited bandwidth and thus efficient pump utilization is not easily achieved under such conditions. That is to say, HBr is not continuously tunable within a broad band, thus as a pump for N2O it offers, at low to moderate pressure, no more than limited line matching. In addition, the 1→0 transition of HBr occupies the spectral range 2400 cm−1 to 2700 cm−1, which presents essentially no spectral overlap with the 00°0→00°1 N2O transition being pumped. The forgoing has direct implications in regards baseline useful emission from HBr, and thus global system efficiency whether or not N2O is at pressure. Further deficiencies, common to this cited prior art and those cited below are presented in the final paragraphs of this section.
The article, Optically Pumped 33 atm CO2 Laser [T. Y. Chang et al, 23, 1973, 370], describes single nanosecond laser pulses at wavelengths near 10 μm that have been obtained by using a pulsed HBr laser to optically pump pure CO2 gas at pressures up to 33 atm in a 1 mm-long optical resonator. At pressures above 17 atm, the laser oscillates on the 10.3 μm R branch rather than on the usual 10.6 μm P branch.
The primary deficiency is, as per the prior cited patents and article, the use of a Hydrogen Bromide laser pump which, in practice, is not suitable. Chemical lasers, though in general efficient, require the handling of exhaust product and precursor fuel and oxidizer. Even if purely cold-reaction discharge initiated by dissociation of a halogen donor, these are then typically inefficient and still require product gas handling. In addition, the 1→0 transition of HBr occupies the spectral range 2400 cm−1 to 2700 cm−1, which presents essentially no spectral overlap with the 00°0→00°1 CO2 transition being pumped. The forgoing has direct implications in regards baseline useful (for any isotopologue of CO2) emission from HBr, and thus global system efficiency whether or not CO2 is at pressure. Further deficiencies, common to this cited prior art and those cited below are presented in the final paragraphs of this section.
The article, Optically Pumped 16 μm Laser [R. M. Osgood, Applied Physics Letters, 28, 1976, 342] describes a potentially useful 16 μm CO2 laser, oscillating on the [10°0,02°0] to 0110 transition, is described. The v=1→v=0 (v is the vibrational level quantum number designator) lines from an HBr chemical laser were used to pump a low-pressure mixture of HBr and CO2 gases. Vibrational energy transfer from HBr followed by a 9.6 μm stimulating pulse populated the CO2 [10°0,02°0] level.
The primary deficiency is, as per the prior cited patents and article, the use of a Hydrogen Bromide laser pump which, in practice, is not suitable. Chemical lasers, though in general efficient, require the handling of exhaust product and precursor fuel and oxidizer. Even if purely cold-reaction discharge initiated by dissociation of a halogen donor, these are then typically inefficient and still require product gas handling. In addition, HBr is a multiline molecular laser with a defined emission line structure, thus at approximately atmospheric pressure or less the CO2 attributable absorption line structure is only in sporadic proximity to a limited number of HBr lines, both CO2 and HBr of limited bandwidth and thus efficient pump utilization is not easily achieved under such conditions. That is to say, HBr is not continuously tunable within a broad band, thus as a pump for CO2 it offers, at low to moderate pressure, no more than limited line matching. In addition, the 1→0 transition of HBr occupies the spectral range 2400 cm−1 to 2700 cm−1, which presents essentially no spectral overlap with the 00°0→00°1 CO2 transition being pumped. The forgoing has direct implications in regards baseline useful (for any isotopologue of CO2) emission from HBr, and thus global system efficiency whether or not CO2 is at pressure. Further deficiencies, common to this cited prior art and those cited below are presented in the final paragraphs of this section.
The preceding approaches are all deficient relative to that of this proposal by virtue of this proposal's specific utilization of laser diode pumped Tm solid state as the source for the direct optical pumping of CO2, and derivative aspects thereof. Continuous tuning has been demonstrated from ˜1.74 μm through ˜2.017 μm, which well matches the CO2 pump transition (00°0→20°1) and (00°0→12°1) range of ˜1.949 μm to 2.035 μm at least inclusive of most to all isotopologues. Laser diode pumped Tm solid state systems have demonstrated impressive performance. Raised pressure operation of said CO2 systems would render locking of Tm system spectral output on pressure broadened 00°0→20°1 transition technically simple. It has been argued that use of optical pumping of molecular transitions necessarily results in reduced performance from the pump source system because of need to narrow the line width of these sources to match the line width of the molecular transition concerned. This in turn depressing global system performance. In the case of high pressure CO2 the pump transition, 00°0→20°1, coalesces into a band of width ˜1000 GHz to 2000 GHz (or 12 to 24 nm) per isotopologue courtesy of pressure broadening at 5 atms for example. Similarly the 00°0→12°1 transition coalesces into a contiguous absorption band of useful extent. Thus in this case this objection does not apply. Secondly, at high dopant level concentrations cross relaxation in Tm media is rapid and the pump pulse event duration required is typically in the region of ˜200 ns full width half maximum which is substantial, and finally it is possible to amplify several spectrally separated wavelength components in a medium which has inhomogeneous line broadening character, thus enhancing effective interaction bandwidth and thus system performance. This aspect is consistent with the one methodology presented here of common amplification of a number of lines for pumping of distinct isotopologues—or a unitary isotopologue on multiple rotational vibrational transitions of the 00°0→20°1 and/or 00°0→12°1 combination bands. Another laser diode (LD) pumped solid state option with face value potential is divalent Cr2+ ion (Chromium) doping. However, the very limited excited state lifetime available is a significant negative factor as solid state system energy storage is not possible absent a secondary LD pumped gain switched optical pump system for this medium. More complex, less efficient and less desirable.
The preceding approaches are deficient in that they do not identify that one of the probable metastable levels likely to be populated post and during the optical pump of the 20°1 band is the 0111 level of similar lifetime to the 00°1 level. Lasing from this level on the transition(s) 0111→1110 and 0111→0310 will display twice as many lines as to be found in the traditional 00°1→10° and 00°1→02°0 transition(s)—this a result of elimination of double degeneracy present for zero angular momentum case, and resulting in continuous tunability of system at half the pressure required for zero angular momentum excited level CO2. Similar arguments apply in regards pumping of the 12°1 level.
The preceding approaches are deficient in that optically pumped raised pressure multi CO2 isotopologue, symmetric and asymmetric, gas mix operation is not identified as a solution to the specific applications of remote sensing and sub picosecond pulse amplification courtesy of the impressively broad, and reproducible, spectrally contiguous gain spectrum resulting therefrom. This enhanced by probable 0111 metastable contribution under appropriate conditions. Optical pumping has the added benefit, if applied appropriately, of not driving significant dissociation in gas and thus can preserve and sustain predefined gas premixes.
Traditional discharge, radio frequency (RF) or e-beam pumping is deficient for a number of interrelated issues. Firstly, they are generally associated with dissociation of the gas mix, and thus require catalyzers for gas recovery. In the case of RF it is restricted to low pressure CO2 operation and is thus only sensible within the Continuous Wave (CW) or quasi CW operational regime. Real gain switched energy/power is not feasible. In the case of high pressure operation only transverse discharge or e-beam applications are sensible in any way; however the discharge voltage, switching requirements and pulse forming network strictures render this difficult and impractical in most applications and reliability is typically a serious issue. This kind of high voltage/high current switching also carries with it the requirement for high voltage supplies and electro magnetic interference (EMI), which are derived from the high voltage/high current discharge events.